The field of this invention is galvanic deposition of palladium or palladium alloys using aqueous plating baths.
In electrical engineering, palladium coatings produced by galvanic deposition are used as favorably priced alternatives for galvanic hard gold coatings on contact components, such as plug connectors, for example. For this application, the palladium coatings must have uniformly semi-bright to bright surfaces, and be sufficiently ductile and pore-free. The baths used for galvanic deposition have to be of simple composition and easy to top up so as to ensure uniform quality in the coatings deposited.
Galvanic baths producing coatings which meet the named conditions operate in the alkaline pH range (e.g., over pH 7) mostly at high temperature, and contain substantial quantities of ammonia which has to be present mainly to form the amine complexes needed for deposition.
With these known baths, the constantly evaporating ammonia causes considerable difficulties in controlling the pH value. Ammonia has to be added continuously in metered quantities for the optimum pH range to be maintainable. In addition, the unpleasant and annoying odor of the evaporating ammonia makes the use of a ventilator means necessary. One further disadvantage is that, in the case of partial coating with palladium, basic material containing copper is corroded by the ammonia vapors, and the corrosion products can contaminate the bath, and thereby change the technological properties of the deposited coatings for the worse.
A known bath for galvanic deposition of palladium nickel alloys also contains aminoacetic acid, but here, through the addition of ammonia, an alkaline pH value is set at which the palladium is present as an ammine complex and not as an aminoacetic acid complex (Isbekowa D. V., Kudra O. K. Tschelikidi V. V., USSR, Saschtschita Metallow, vol. IX No. 1/73, pp 108-110).